The present invention relates to the field of test equipment, and more particularly to a hand-held probe to sense the presence of an alternating current (AC) signal voltage on a conductor.
Hand-held electrical probes are known in the art for detecting the presence of AC signal potential on a conductor. The probes are either of a contacting type which requires direct electrical contact with an uninsulated portion of the conductor, or of a noncontacting type which senses the resulting electromagnetic field when placed in proximity to the conductor. There is a safety risk associated with the contacting type probe due to the possibility that the user may receive a harmful electrical shock, so that the noncontacting type probe is preferable.
Since both type probes use a visual and/or audible indicator to annunciate the presence of a detected voltage potential to a user, many of the probes have manual on/off switches that allow a user to deactivate the probe when not in use. The switch, however, presents a number of disadvantages. If the switch is made of metal, it can act as a conduit to shock and injure the user if the switch comes in contact with a xe2x80x9clivexe2x80x9d wire or circuit. A user may forget to turn the switch on before using it, thereby risking a false negative reading, or a user may forget to turn it off causing the batteries to run down. A manual on/off switch can also break with repeated usage over an extended period of time. Therefore, it is preferable for a user to have a probe that does not rely on the use of a manual on/off switch.
U.S. Pat. No. 5,103,165 discloses an improved, switchless type probe that may be contacting or noncontacting. In a best mode embodiment the ""165 probe housing comprises non-conductive material, such as polyvinylchloride (xe2x80x9cPVCxe2x80x9d), to reduce the risk of electrical shock for a user, and it eliminates the use of a switch through the use of internal circuitry that automatically activates the probe when it senses an electrical current or voltage, thereby reducing the drain on the batteries. The probe disclosed in the ""165 patent provides a visual indicator to alert the user to the presence of an electrical current or voltage, but does not have an audible indicator. The insulated housing of the ""165 probe, however, presents a problem with static charge build-up, which causes flickering of the visual indicator and would cause chirping of an audible indicator. The static build-up may occur when the probe is rubbed on cloth, such as a user""s pocket or sleeve. This intermittent flickering and/or chirping may be annoying to the user, and it would also drain the batteries.
The present invention is to a switchless, hand-held probe for detecting and annunciating the presence of AC voltage signals on a conductor, which incorporates filtering circuitry that minimizes intermittent activation of the probe""s indicator due to static charge build-up. The probe of the present invention comprises an antenna adapted to sense the electrical energy radiated by the AC voltage on the conductor, detector circuitry responsive to the sensed electrical energy at the antenna for providing an electrical signal when the electrical energy at the antenna satisfies certain magnitude and duration thresholds, an indicator to alert a user to the presence of the sensed electrical energy, and activation circuitry connected to the detector circuitry and adapted to activate the indicator only when a sufficient number of signals are received from the detector circuitry during a period of time, thus minimizing the intermittent activation of the probe""s indicator due to static charge build-up and any associated drain on the batteries thereby.
In the present hand-held probe, the detector circuitry establishes energy thresholds concerning the electrical energy sensed by the antenna. The detector circuitry filters both the magnitude and frequency of the sensed electrical energy. When the sensed electrical energy satisfies the energy thresholds, an electrical signal, which may vary in frequency and duration, is produced by the detector circuitry and received by the activation circuitry. The activation circuitry functions as a switch by activating the indicator only when a sufficient number of signals are received from the detector circuitry during a given period of time. If enough signals are not received by the activation circuitry from the detector circuitry, the indicator will not be activated.
In one embodiment of the invention, the probe includes an antenna adapted to sense the electrical energy radiated from a conductive member when the probe is positioned adjacent the member; an indicator adapted to alert a user when activated; detector circuitry comprising a first inverter, connected to the antenna, adapted to respond to a range of voltages sensed on the antenna, and a second inverter, connected to the first inverter, adapted to respond each time the output voltage of the first inverter changes; and activation circuitry comprising a capacitor, connected to the second inverter and indicator, adapted to charge on receipt of the signal from the second inverter for activating the indicator when the capacitor is charged sufficiently, and an oscillator adapted to periodically discharge the capacitor.
These and other objects, features, and advantages of the present invention will become more apparent in light of the following detailed description of a best mode embodiment thereof, as illustrated in the accompanying Drawing.